1. Field of the Invention
The present invention relates to an exhaust system for an internal combustion engine having multiple-cylinders. Specifically to a system which includes a confluent exhaust passage converging at least two exhaust passages upstream thereof, each upstream exhaust passage being communicated separately with associated plural branched exhaust passages of an exhaust manifold communicating the exhaust ports of a multiple-cylinder engine.
2. Description of the Prior Disclosure
Recently, there have been proposed and developed various exhaust systems including at least two intermediate exhaust passages, each communicated separately with the associated plural branch exhaust passages of an exhaust manifold. In general, the intermediate exhaust passages are formed of a dual exhaust tube.
One such conventional exhaust system is shown in FIG. 1. Referring now to FIG. 1, a four-cylinder in line engine includes a cylinder head 1 having exhaust ports through which first, second, third and fourth cylinders #1, #2, #3 and #4, respectively communicate branch exhaust passages a, c, d and b of an exhaust manifold. The exhaust system includes a dual exhaust tube section 2 defining first and second exhaust passages 2.sub.a and 2.sub.b. The first exhaust passage 2.sub.a communicates both downstream ends of the passages a and b, while the second exhaust passage 2.sub.b communicates both downstream ends of the passages c and d. The two exhaust passages 2.sub.a and 2.sub.b are communicated with a single confluent exhaust passage 8 through a nodal point or confluent point 3. As is generally known, a catalytic converter or a muffler assembly 4 is employed on an exhaust tube defining the exhaust passage 8 to absorb and damp out exhaust noises or to convert gaseous pollutants into harmless gases.
Another conventional exhaust system including an intermediate dual exhaust tube connected to a plurality of branch exhaust pipes of an exhaust manifold has been disclosed in Japanese Utility Model First Publication (Jikkai) Showa 63-79439. The prior art exhaust system further includes a pulse converter 5 downstream of the intermediate dual exhaust tube 2 as shown in FIGS. 2 and 3. The exhaust system shown in FIG. 2 is different from that of FIG. 3 in that the exhaust system is applied to internal combustion engines with a turbocharger 9 (a supercharger driven by the engine exhaust gas).
As clearly seen in FIGS. 2 and 3, an ejector 51 is provided upstream of and adjacent to the confluent point 3 such that its cross section is gradually choked towards its downstream direction. A diffuser 53 is also provided downstream of and adjacent to the confluent point 3 such that its cross section is gradually expanded towards its downstream direction. As is well known, the ejector 51 acts for converting fluid having high pressure and low speed to fluid having low pressure and high speed, while the diffuser 53 acts for converting fluid having low pressure and high speed to fluid having high pressure and low speed. The aforementioned pulse converter 5 is composed of the ejector 51 and the diffuser 53 as best seen in FIG. 4. Traditionally, the pulse converter 5 is utilized to improve engine torque within a low or medium engine speed range.
Such conventional exhaust systems have various problems. These problems will be described in detail by comparing two engine performance curves shown in FIG. 5, one (as shown in the phantom line q of FIG. 5) being a performance curve of a four-cycle in line engine utilizing the exhaust system without the pulse converter as shown in FIG. 1, the other (as shown in the continuous line p of FIG. 5) being a performance curve of a four-cycle in line engine utilizing the exhaust system with the pulse converter as shown in FIG. 3.
As appreciated from the graph of FIG. 5, it will be found that, within a medium or low engine speed range, the engine torque of the exhaust system having the pulse converter exceeds that of the exhaust system not having the pulse converter because exhaust gases flowing through the two passages 2.sub.a and 2.sub.b are converted in such a manner as to have higher speed and lower pressure by the ejector 51 and thus exhaust gas flowing through one of the passages 2.sub.a and 2.sub.b does not disturb gas flow through the other but facilitates its exhaust gas flow. In this manner, when the engine is running at a low or medium speed, the ejector 51 acts to prevent exhaust gas flows through both passages 2.sub.a and 2.sub.b from interfering with each other and additionally to enhance charging efficiency of the air-fuel mixture and scavenging efficiency of exhaust gas in such a manner that the exhaust gas flowing through one of the passages 2.sub.a and 2.sub.b is effectively exhausted with the aid of gas flow through the other passage in jet fashion, during blow-down. Such effect of the ejector will be hereinafter referred to as an "ejector effect".
On the other hand, in the exhaust system of FIG. 4 having the pulse converter, when the engine is running at a high speed, exhaust gas flow through the ejector 51 is choked in the vicinity of the outlet 52 of the ejector 51 due to excessively great exhaust gas flow. This results in increase in exhaust pressure upstream of the outlet 52. Consequently, back pressure rises and therefore exhaust loss is increased, thereby lowering the charging efficiency of the mixture. For this reason, as seen in the graph of FIG. 5, the engine torque of an exhaust system not including a pulse converter exceeds that of a system including a pulse converter, at high engine speeds.
In other words, the conventional exhaust systems including a pulse converter are so designed that engine torque at low or middle engine speeds is enhanced at the sacrifice of engine torque at high engine speeds.
Although the exhaust systems not including a pulse converter have the advantage of generating higher engine torque during engine high speeds when compared with the exhaust systems including a pulse converter, there is a tendency for choked flow to occur at the confluent point of the two passages 2.sub.a and 2.sub.b during excessively high engine speeds, thereby resulting in lowering of engine torque.